Allergies  to foods can be  acute and the symptom onset rapid, sometimes within moments. The biological cause is very well understood and because of the rapid  symptom onset, testing is usually not necessary to identify the culprit.  These reactions are not very common but if you think you have a "true" food allergy it is wise to visit an allergist for comprehensive testing and advice on precautions.
More commonly, delayed adverse reactions to foods, additives and other chemicals are not really classical allergies and the delay of symptom onset and the multitude of possible pathogenic mechanisms involved render the identification of the offending food(s) or substance(s) to be much more complicated. Whereas approximately only 2-5% of the US pop. has a classical allergy to a food it is estimated that as many as 80-90% of the population  has some form of adverse reaction to one or more foods or additives manifesting in a broad range of disorders; including, migraine headache, weight gain, fatigue, hyperactivity/ADD, arthritis, breathing and skin disorders, recurring ear infections in children, depression and various others.

Standard allergy tests, such as skin testing, are not accurate for these type of reactions as they measure only a single mechanism, such as mast cell release of histamine or the presence in the blood of the IgE molecules associated with such release.

 

Why we're sensitive to some foods.
The immune system is an intricate structure which, under normal circumstances, protects the body from the harmful effects of invading elements - viruses, bacteria, other micro-organisms and toxins. However, sometimes the system over reacts to substances, including foods and food additives, that are not normally harmful. The result: hay fever, asthma, migraine, skin problems, intestinal disorders, painful joints or muscles, frequent ear infections and hyperactivity in children, chronic fatigue; and, along with any of these or other disorders, weight imbalance. What makes identifying these harmful foods difficult is that the symptoms may not emerge until 8 -72 hours after eating or may follow a dose/frequency dependence that needs several feedings over several days to weeks.

Is this a common problem? Yes! These conditions impact an estimated 75 - 90% of the population; costing billions in treatments, lost income due to missing work and absenteeism from school. Former US Surgeon General, C. Everett Koop, regards obesity as the number 2 cause of death in America. Obesity substantially increases the risk of heart disease, osteoarthritis, diabetes, certain cancers and other disorders.

Further, it’s a scientific fact that appetite and metabolism are controlled by blood chemistry, which is adversely affected when an offending food is consumed. Food sensitivities also cause cravings for sweets and starches as well as water retention. Therefore, knowing and avoiding the foods you react to can help you reach your desired weight and feel much better without significant cost or the use of prescription drugs.

Whether you have a particular food allergy or several allergies, only your physician can tell you for sure, but you need to be aware than something as simple as a food allergy can be a primary cause of your weight gain and struggle to lose the weight.